A conventional pacemaker stimulates a patient's heart to maintain regular contractions of the heart thereby promoting blood circulation within the patient. Such stimulation may be prescribed when the patient's heart does not function normally due to, for example, a genetic condition.
In a healthy heart, contractions occur first in the muscles associated with the atria of the heart, followed by contractions in the muscles associated with the larger ventricles of the heart. In this way, atria chambers assist in the filling of ventricle chambers with blood returning from the veins. This enables the ventricles to more efficiently pump blood to the arteries.
Given the interaction of these chambers, efficient operation of the heart is predicated on each of the chambers operating in a proper timing sequence and having contractions that pump a sufficient amount of blood from each chamber. For example, during contraction the right atrium chamber should pump enough blood to effectively “fill” the right ventricle chamber. Moreover, this should occur immediately before the right ventricle begins to contract. In this way, the heart may efficiently pump blood on a repetitive basis.
A healthy heart repetitively contracts in the above described manner in response to the generation and conduction of electrical signals in the heart. These electrical signals are generated in and conducted through the heart during every beat of the heart.
Under certain circumstances, a pacemaker may compensate for abnormal operation of a heart by pacing (e.g., stimulating) one or more of the atria and/or ventricles. To stimulate the heart, a typical pacemaker generates a series of electrical signals which are applied to the heart via one or more electrodes implanted in the heart (e.g., in ventricular or atrial chambers). These electrical signals cause the heart to contract in much the same way as the native electrical signals discussed above cause the heart to contract.
To provide appropriate timing for the generation of electrical signals, conventional pacemakers may sense signals in the heart. For example, a pacemaker may sense electrical signals in the atria to detect when the atria are being activated. The pacemaker may then delay a prescribed period of time after which it senses electrical signals in the ventricles or atria to determine whether to apply a stimulus to the ventricles or atria. In this way, the pacemaker may stimulate the ventricles or atria at the appropriate time, if necessary, in an attempt to maintain efficient operation of the heart.
The signals from the sensors in the heart may be collected in the pacemaker to be further analyzed to determine when a stimulus should be generated or other action taken. For example, the data received from the sensors may be stored as a time series of data. The time series includes an indicator of the time a sample of data was taken and a value such as amplitude of the signal at that time. This time series data may then be processed by an appropriate circuit and/or analyzed by morphology detection algorithms and arrhythmia detection algorithms to determine the timing of the electrical pulses to pace the heart.
The appropriate pulse generation timing is not dependent entirely on the detection of other cardiac signals. That is, optimum timing may not result from simply detecting the activation of the atria and then waiting a prescribed amount of time before generating a pulse to stimulate the ventricles. The timing and characteristics of the electrical signal in the heart are affected by the overall state of the heart and the activity of the patient. For example, the timing and characteristics of signals in the heart differ between an awakened state and a sleep state and between a resting state and an active state. Correct tracking and determination of the state of the patient improves the efficiency and accuracy of the signaling of the pacemaker and consequently improves the function of the heart.